A power supply system in which a battery feeds power to a load is installed in a vehicle. Power supply systems in which a battery feeds power to a load include a power supply system including two semiconductor switches whose respective one ends are connected to each other. In such power supply systems, the other end of one of the semiconductor switches is connected to the positive electrode of the battery, and the other end of the other semiconductor switch is connected to one end of the load. Then, the current flowing from the battery to the load is controlled by substantially simultaneously turning on or off the two semiconductor switches.
JP 2014-49686A discloses a current control device that substantially simultaneously turns on or off two semiconductor switches whose respective one ends are connected to each other, thereby controlling the current flowing between the respective other ends of the two semiconductor switches. The current control device described in JP 2014-49686A includes two N-channel FETs (Field Effect Transistors), and each of the two FETs functions as a semiconductor switch.
For the two FETS, the source of one FET is connected to the source of the other FET. A common voltage is applied to the respective gates of the two FETs. The two FETs are substantially simultaneously turned off or on by adjusting the voltage applied to the respective gates of the two FETs. Consequently, the current flowing through the respective drains of the two FETs is controlled.
When a current flows through two semiconductor switches, heat is generated from each of the two semiconductor switches. The amount of heat generated from a current control device including two semiconductor switches whose one ends are connected to each other increases with an increase in the resistance between the other ends of the two semiconductor switches, or in other words, the combined resistance of the on resistances of the two semiconductor switches, and increases with an increase in the value of the current flowing via the two semiconductor switches. When a large amount of heat is generated between the two semiconductor switches, there is the possibility that a short circuit may occur between the other ends of the two semiconductor switches.
Today, a large number of loads that are powered by a battery are installed in a vehicle, so that it is necessary to supply a large current to the large number of loads via the two semiconductor switches. Therefore, as a current control device that controls a current by substantially simultaneously turning on or off the two semiconductor switches, there is a need for a current control device that generates less heat even when a large current flow through the two semiconductor switches.
As a current control device that generates less heat, it is conceivable to use a current control device including two semiconductor switches having small on resistances.
However, for semiconductor switches having the same breakdown voltage thereacross, a semiconductor switch having a small on resistance is generally small. Therefore, a current control device including two semiconductor switches having the same breakdown voltage thereacross and having a small on resistance is large, and thus is not suitable as a current control device installed in a vehicle, which has a limited space.
Furthermore, since large semiconductor switches are expensive, a current control device including two semiconductor switches having a small on resistance also has the problem of an increased manufacturing cost.
The present invention has been made in view of such circumstances, and it is an object of the invention to provide a compact current control device that generates less heat and can be manufactured inexpensively, and a power supply system including the current control device.